Fusible element arrangement for fuse

ABSTRACT

A low ampere current limiting fuse includes a fusible element spirally wound on an elongated, cruciform in cross section support and carrying an M-spot. The turns of the fusible element are wound on a smaller or tighter pitch in the area of the M-spot to locate a relatively large number of turns in the area of the M-spot so that heat generated in the fusible element is concentrated in the area of the M-spot. An indicator wire is also spirally wound on the cruciform support and passes between the smaller pitch turns and the support. A cement holds the indicator wire away from the fusible element and also engages and holds the smaller pitch turns.

United States Patent Mikulecky [54] FUSIBLE ELEMENT ARRANGEMENT- FOR FUSE [72] Inventor: Harvey W. Mlkulecky, Racine, Wis.

[73] Assignee: McGraw-Edison Company, Milwaukee, Wis.

[22] Filed: Nov. 30, 1970 [21] Appl. No.: 93,679

' [451 Dec. 19, 1972 FOREIGN PATENTS OR APPLICATIONS 607,381 8/1948 Great Britain ..337/166 Primary Examiner-Bernard A. Gilheany Assistant Examiner-Dewitt M. Morgan Attorney-John W. Michael, Gerrit D. Foster, Bayard H. Michael, Paul R. Puerner, Joseph A. Gemignani, Robert E. Clemency, Andrew 0. Riteris, Glenn A. Buse and Spencer B. Michael [57] ABSTRACT is also spirally wound on the cruciform support and passes between the smaller pitch turns and the support. A cement holds the indicator wire away from the fusible element and also engages and holds the smaller pitch turns.

9 Claims, 3 Drawing Figures [52] US. Cl. ..337/163, 337/166, 337/296 [51] Int. Cl. ..H01h 85/06, H0111 85/14 [58] Field of Search ..337/159, 160, 293, 296, 163, 337/166 [56] References Cited UNITED STATES PATENTS 3,425,019 1/1969 Kozocka ..337/16 3 3,569,891 3/1971 Cameron "337/293 X 3,140,371 7/1964 Johann et al. "337/2963; 2,808,487 10/1957 Jacobs, Jr. ..337/29 6 X 3,483,502 12/1969 Mikulecky.... .....337/159 X 1,227,752 5/.l917 Cole ..337/293 '34 0 II".\ \l

PATENTEnnEmmn 3.706.951

SHEET 1 BF 2 1 FUSIBLE ELEMENT ARRANGEMENT FOR FUSE BACKGROUND OF INVENTION This invention relates to fuses and, more particularly, to an arrangement of the fusible element in the fuse.

Current limiting fuses, or fuses generally, are well known as is the use of M-spots on the fuse elements. The M-spots are connected to the fusible element and are effective to alloy with the fusible element above a given temperature to produce an alloy which will fuse below the fusion temperature of the fusible element per se.

Fuses generally have an ampere rating which expresses their normal current carrying ability. The conventional approach to lowering the ampere rating is to reduce the diameter, or cross sectional area, of the fusible element. For example, where the fusible element takes the form of a wire the diameter of the wire is reduced to achieve lower ampere ratings. This approach, however, is also accompanied by compensating problems. The smaller diameter wires are more susceptible to blowing (fusion) under high inrush currents (e.g. transformer inrush currents), are more difficult to assemble, and are more susceptible to damage due to vibration or mechanical shock. I

SUMMARY OF INVENTION Among the generalobjects of this invention is to increase the rating of a fuse, more specifically to lower the ampere rating of a current limiting fuse.

For the achievement of these and other objects, this invention proposes an arrangement wherein the fusible element of a, fuse, preferably a current limiting fuse, carries an M-spot and is supported such that a portion of the fusible element is bunched in the area of the M- spot. Where the fusible element is wound in spiral form,it is wound in the area of the M-spot on a smaller pitch to locate a larger number of turns adjacent the M- spot. This concentrates heat in the area of the M-spot and affects its operation. The M-spot will thus operate at a lower ampere rating without reducing the wire diameter so that inrush current capability and mechanical strength are not sacrificed.

An indicator wire, which is conventional in many ,fuse constructions, can also'be used. Where used, the

indicator wire is directed past the smaller pitch turns and held away from the fusible element by a cement. Preferably, the cement is also arranged to engage and hold the smaller pitch turns.

DESCRIPTION OF DRAWINGS FIG. 1 is a side view of a portion of a current limiting fuse embodying in this invention;

FIG. 2 is a section view taken generally along line 2--2 in FIG. 1; and

FIG. 3 is a diagram illustrating a number of curves plotting average melt time-current characteristics for fuses embodying this invention.

DESCRIPTION OF PREFERRED EMBODIMENT With particular reference to the drawing, a portion of a fuse is illustrated in FIG. 1, the construction illustrated being that which is commonly embodied in a current limiting fuse. Structurally, a support having -a cruciform cross section is generally elongated and is provided with electrode assemblies 12 and 14 at the opposite ends thereof. Support 10 includes four radially extending arms 16, 18, 20, and 22 with adjacent arms having spaces 24, 26, 28, and 30 therebetween. A fusible element 32 is wound on the support and extends between electrodes 12 and 14, being electrically connected to both electrodes. In accordance with conventional practice, an indicator wire 34 is also wound on the support. Generally, the structure of the fuse is completed by providing an outer electrical insulating tubular housing to enclose the structure just defined and in some instances the interior of the fuse may also be filled with an arc quenching material such as quartz sandhThe sand completely surrounds the cruciform support, filling openings 24, 26, 28, and 30, and filling any spaces between the support and the interior wall of the vfuse tube. The fuse is also provided with exterior hardware to permit its assembly into a conventional fuse cut-out arrangement or the like. Since the complete external details of the fuse are not necessary to a complete understanding of this .invention, they have neither been illustrated nor will they be described.

In accordance with conventional practice an M-spot 36 is carried on fusible element 32. In operation, the fusible element has a particular ampere current carrying capacity above which the heat generated will cause the fusible element to fuse initiating the interruption and current clearing operation. The purpose of the M- spot is to afford better control over the initiation of this interrupting operation. As is well known, the M-spot is generally made of a material which will alloy with the material of fusible element 32 under the influence of exposure to a particular temperature. That temperature is generally selected to be below the fusion temperature for the fusible element 32 per se and the fusion temperature of the resulting alloy is generally selected such that fusion will start in the M-spot area after the alloy has been formed and before the entire fusible element reaches its particular temperature. Thus, fusion initiates at the M-spot.

Generally, there have been limitations on lowering the ampere rating of fuses of this type. The common approach has generally been to reduce the cross sectional area of the element, i.e. the diameter in case of a wire fusible element, to achieve an arrangement wherein interruption will occur at a lower ampere rating. However, various problems have also been attendant this type of approach. First, the smaller elements are more susceptible to fusion as a result of high inrush currents, for example transformer inrush currents, and secondly they do not exhibit desirable mechanical strength. Such smaller diameter wires are generally more difficult to assemble and also are more susceptible to damage from vibration and/or mechanical shock.

It has been discovered that the ampere rating can be lowered in another manner and one which obviates the necessity for using the smaller element sizes. In accordance with this invention, it is proposed to produce a bunching of fusible element in the area of the M -spot. By so doing, a larger amount of fusible element is concentrated in the area of the M-spot thereby increasing the amount of self-generated fusible element heat to which the M-spot is exposed. This concentration of heat causes the M-spot to operate at a lower current condition thereby affording a lower ampere rated fuse.

I060ll OlZO More specifically, in the illustrated embodiment the fusible element is wound in spiral form and is supported in that form within the fuse tube. As illustrated, the fusible element is wound over cruciform support 10 engaging-the outer ends of legs 16, 18, 20, and 22. In the area of M-spot 36, the winding of the fusible element is achieved with a closer spacing between adjacent turns. This closer spacing is preferably provided in front of as well as after the M-spot, considering the winding as proceeding from electrode 12 to electrode 14. The bunching ofthe turns of the M-spot can be achieved by,

starting from electrode 12, winding the fusible element at a constant relatively large pitch to a point adjacent the'area of the M-spot, this being labeled in the drawing as extension A. From that point, the turns are made on a smaller pitch beyond the M-spot to a point where the relatively larger, constant pitch is resumed, the smaller pitch being designated as extension B and the resumption of the constant pitch as extension C. As an exam ple, the pitch through area A can be approximately one thirty-secondof an inch and the pitch through area C is resumedat 'eleven-sixteenths of an inch. Generally,

the same number of turns are provided before and after the M-spot and it is usually desirable to havea slightly larger spacing between the center turn and the two adjacent turns on either side, for example one-sixteenth of an inch where the smaller pitch through area B is one thirty-second of an inch. In the example illustrated in the drawing, 10 one-half turns are provided through area B.

The smaller pitch turns are then adjacent to the area of the M-spot and also in heat transfer relation with the M-spot to influence the operation of the M-spot in initiating fuse operation. It has also been found that with this arrangement, higher are voltages are not generated in the fuse as might be expected because of the relative- 1y longer element length which is used. It was discovered that the close spacing between the bunched turns results in flash-over between the turns thereby providing an arc length which is less than the-overall element length and does not result in an increase in the arcvoltage.

With this arrangement, lower ampere rated fu'ses can be achieved without sacrificing other desirable characteristics. FIG. 3 is a plotting of three curves, X, Y, and Z. The curves plot current in amperes against average melt time in seconds. Curve X corresponding to 1.5 ampere rated fuse, curve Y corresponding to a 3.0 ampere rated fuse, and curve Z corresponding to a 4.5 ampere rated fuse, all embodying this invention.

As is conventional practice in fuses of this type, an indicator wire 34 is usually provided to provide an external visual indication of when the fuse has been called upon to operate. The indicator wire is generally wound spirally parallel to the main fusible element and bunching the main fusible element turns in the area of the M-spot presents a problem relative to the manner in which the indicator wire is to traverse that area. To solve that problem, this invention proposes that the indicator wire 34 be wound spirally parallel to the fuse element 32 to a point 38 which is in front of extension B. From point 38, the fuse element proceeds in a I eleven-sixteenths of an inch, the pitch through area'B is 1 generally axial direction through opening 28 past the 6 bunched turns, i.e. between the bunched turns and the cruciform support see FIG. 3, to an eyelet 40 anchored in the leg 20. This manner of anchoring the indicator wire part way along the extension of the fuse is described and claimed in U.S. Pat. No. 3,483,502 of Harvey W. Mikulecky and assigned to the assignee of this invention. It will also be appreciated that rather than terminate the indicator wire at eyelet 40, having passed the bunched turns, the spiral winding parallel to fuse element 32 can be resumed and the indicator wire can proceed to the .opposite end of the fuse. As described in the aforementioned patent, a conventional indicator mechanism (not shown) is provided in the area of electrode 12.

To avoid shorting the bunched turns, the indicator wire is held away from, or electrically isolated from, the bunched turns by a body of cement 42. The cement holds the indicator wire closely adjacent the inner wall of the cruciform support opening 28. It has been found to be advantageous to provide, a sufficient quantity of cement such that the .cement will also engage the bunched turns and hold the turns in, the desired spaced relationship. Itwill be noted in FlG. l that'the' cement has an axial extension which is greater than theaxial extension of the bunched turns, and in FIG. 3 that the cement extends radiallyto engage'the bunched turns of the fusible cement.

Although'but one'embodiment of the present invention has been illustrated and described, it will be apparent to those skilled in the art that various changes and modifications may be made therein without depart ing from the spirit of the invention or from the scope of the appended claims.

lclaim: g I 1. A fuse arrangement comprising, in combination, spaced terminal means, H a fusible element connected to and extending between said terminal means, support means engaging and supporting said fusible elementin spiral form, M-effect causing means carried by said fusible element, and in the area of 'said' M-effect causing means a number of turns of said'fusible element having a relatively small pitch therebetween to provide a number of closely spaced fusible element turns adjacent and in heat transfer relation with said M-effect causing means. 2. The fuse arrangement of claim 1 wherein a portion of the turns of said fusible element on both sides of said M-effect causing means have said relatively smaller pitch therebetween. I

3. The fuse arrangement of claim 1 wherein the turns of said fusible element have a first generally constant pitch fromone of said terminal means to the area of said M-effect causing means, the turns of said fusible element in the area of said M-effect causing means having a smaller Pitch, and the turns from said area to the other of said terminal means to which said fusible element is connected have a generally constant pitch greater than said smaller pitch.

4. The fuse arrangement of claim 1 including an indicator wire extending in spiral form along said support means to the area of said M-effect causing means and in said area extending across said smaller pitch turns,

and means maintaining electrical isolation of said indicator wire from said fusible element in the area of said M-effect causing means.

5. The fuse arrangement of claim 1 wherein said support means is an elongated body of 5 electrical insulating material having a cruciform shape, wherein said fusible element engages the arms of said support means,

including an indicator wire spirally wound on said support in engagement with said arms but spaced from said fusible element, said indicator wire extending generally axially and between said support and said smaller pitch turns in the area of said M- effect causing means,

and including cement means holding said indicator wire away from said smaller pitch turns and also engaging and holding said smaller pitch turns.

6. The fuse arrangement of claim 5 wherein a portion of the turns of said fusible element on both sides of said M-effect causing means have said relatively smaller pitch.

7. The fuse arrangement of claim 5 wherein the turns of said fusible element have a first generally constant pitch from one of said terminal means to the area of said M-effect causing means, the turns of said fusible element in the area of said M-effect causing means having a smaller pitch, and the turns from said area to the other of said terminal means to which said fusible element is connected have a generally constant pitch greater than said smaller pitch.

8. The fuse arrangement of claim 1 including an outer housing enclosing said fusible element, said M-effect causing means, and said support means,

and arc-quenching material filling said housing and surrounding said fusible element and said fusible element supporting means.

9. A fuse arrangement comprising, in combination,

a fusible element,

means for connecting said fusible element in an electrical circuit to be protected,

M-effect causing means on said fusible element,

means supporting said fusible element with said fusible element having a first extension up to said M- effect causing means and, in the area of said M-effect causing means, a second extension concentrating a portion of the length and resistance of said fusible element per unit length of said fusible element supporting means adjacent and in heat transfer relation with said M-effect causing means so that the alloying of said M-effect causing means with said fusible element is influenced by said concentration of fusible element length and the increase in generated heat resulting therefrom,

an outer housing enclosing said fusible element, M

effect causing means, and said fusible element supporting means, and arc-quenching material filling said housing and surrounding said fusible element and said fusible element supporting means.

lOll 0122 

1. A fuse arrangement comprising, in combination, spaced terminal means, a fusible element connected to and extending between said terminal means, support means engaging and supporting said fusible element in spiral form, M-effect causing means carried by said fusible element, and in the area of said M-effect causing means a number of turns of said fusible element having a relatively small pitch therebetween to provide a number of closely spaced fusible element turns adjacent and in heat transfer relation with said M-effect causing means.
 2. The fuse arrangement of claim 1 wherein a portion of the turns of said fusible element on both sides of said M-effect causing means have said relatively smaller pitch therebetween.
 3. The fuse arrangement of claim 1 wherein the turns of said fusible element have a first generally constant pitch from one of said terminal means to the area of said M-effect causing means, the turns of said fusible element in the area of said M-effect causing means having a smaller pitch, and the turns from said area to the other of said terminal means to which said fusible element is connected have a generally constant pitch greater than said smaller pitch.
 4. The fuse arrangement of claim 1 including an indicator wire extending in spiral form along said support means to the area of said M-effect causing means and in said area extending across said smaller pitch turns, and means maintaining electrical isolation of said indicator wire from said fusible element in the area of said M-effect causing means.
 5. The fuse arrangement of claim 1 wherein said support means is an elongated body of electrical insulating material having a cruciform shape, wherein said fusible element engages the arms of said support means, including an indicator wire spirally wound on said support in engagement with said arms but spaced from said fusible element, said indicator wire extending generally axially and between said support and said smaller pitch turns in the area of said M-effect causing means, and including cement means holding said indicator wire away from said smaller pitch turns and also engaging and holding said smaller pitch turns.
 6. The fuse arrangement of claim 5 wherein a portion of the turns of said fusible element on both sides of said M-effect causing means have said relatively smaller pitch.
 7. The fuse arrangement of claim 5 wherein the turns of said fusible element have a first generally constant pitch from one of said terminal means to the area of said M-effect causing means, the turns of said fusible element in the area of said M-effect causing means having a smaller pitch, and the turns from said area to the other of said terminal means to which said fusible element is connected have a generally constant pitch greater than said smaller pitch.
 8. The fuse arrangement of claim 1 including an outer housing enclosing said fusible element, said M-effect causing means, and said support means, and arc-quenching material filling said housing and surrounding said fusible element and said fusible element supporting means.
 9. A fuse arrangement comprising, in combination, a fusible element, means for connecting said fusible element in an electrical circuit to be protected, M-effect causing means on said fusible element, means supporting said fusible element with said fusible element having a first extension up to said M-effect causing means and, in the area of said M-effect causing means, a second extension concentrating a portion of the length and resistance of said fusible element per unit length of said fusible element supporting means adjacent and in heat transfer relation with said M-effect causing means so that the alloying of said M-effect causing means with said fusible element is influenced by said concentration of fusible element length and the increase in generated heat resulting therefrom, an outer housing enclosing said fusible element, M-effect causing means, and said fusible element supporting means, and arc-quenching material filling said housing and surrounding said fusible element and said fusible element supporting means. 